Abstract

Hydrothermal vents, characterized by extreme environmental conditions, like high temperatures
and steep geochemical gradients, are considered to be among the most intriguing areas in
marine research. In this study, we investigated a shallow-water hydrothermal system found off
the island of Milos, in the Aegean Volcanic Arc (Eastern Mediterranean Sea). In particular, we
explored the geochemistry, based on porewater analysis and on in situ microsensor
measurements, and the microbial communities, based on molecular fingerprinting techniques, in
the hydrothermal sediment along a temperature gradient towards the center of the vent. We
have observed that on a spatial scale of less than 10 m, from the unaffected to the venting sites,
the sediment geochemistry changed substantially. The in situ microgradients of O2, H2S, pH and
temperature revealed different microenvironments in the hydrothermally impacted sediment,
comprising a variety of potential microniches for diverse microbial communities. However,
differences in the bacterial community structure were not that prominent between the different
venting sites. We found considerably different community structure and composition between
the venting and the unaffected sites, as well as, decrease in operational taxonomic units (OTUs)
richness with increasing hydrothermal activity. We believe that these differences between
community structure and OTU richness are due to the extreme hydrothermal conditions; thus
bacterial communities are selected based on their metabolic capabilities and their ability to
tolerate extreme conditions. Indeed, when we tried to reveal the main factors that control the
microbial communities, sediment geochemistry and especially H2S and pH, were the major
parameters influencing the variation in the bacterial community structure. To test how varying
H2S concentrations and fluid flow velocities affect the biogeochemistry of vent-impacted
sediment, ex situ simulations were performed. Based on lower than expected H2S concentrations
measured in the treated sediment, we assume that in addition to the typical consumption of
sulfide in the oxic-anoxic interface, processes occurring deeper in the sediment (bellow 3 cm) are
responsible for additional removal of sulfide from the system. Overall, several different
geochemical microenvironments were revealed along with a spatial pattern in the microbial
communities of the different hydrothermally impact sediment.